Ink jet printing has already replaced or supplemented in many fields classic image reproduction methods such as photography on silver halide materials, offset printing and screen printing. It is used in office surroundings, in the photographic field, in the graphic field, especially in proofing, in the production of colored textile materials or in industrial environments. In ink jet printing, extremely small ink droplets are expelled through the nozzles of a printing head and are deposited on a recording medium, using digital image data. The inks used therein consist of solutions or dispersions of colored or black dyes or pigments in a liquid vector. The liquid vector consists, depending on the type of the dye or the pigment, of water, of mixtures of water with water-miscible organic solvents, of mixtures of exclusively organic solvents, of oils or of hydrophobic organic solvents.
Ink jet printing has attained a remarkable degree of maturity. For very demanding applications in the graphic field and the photographic field, however, further improvements are necessary. In these fields, the light stability and the volume of the color space (gamut) are extremely important. Both properties are crucially influenced by the constitution of the dyes or pigments that are used. Pigments normally show a better light stability than soluble dyes. In most cases, however, dyes have a higher brilliance and therefore show more brilliant colors. Furthermore, pigments have a tendency to stay on the surface of the printed medium and therefore to reduce the gloss of the surface. This is not desirable especially for photographic applications of ink jet printing.
Up to now, dye based inks offer a considerably better reproduction of brilliant colors. For applications where a high brilliance of the colors is needed, inks containing dyes and not pigments are preferentially used. Generally, the use of dye-based inks also gives a cost advantage, because in most cases dyes are cheaper than pigments.
In ink sets for demanding applications such as in the graphic field, in photography or in proofing applications, the most brilliant available dyes have to be used. These dyes must show, besides good water fastness and high diffusion fastness, an excellent light stability.
The simplest ink set consists of one yellow ink, one magenta ink, one cyan ink and one black ink. The three colored inks (primary colors) allow, by superposition, the reproduction of the secondary colors such as red, blue and green and of all intermediate colors.
Inks for ink jet printing need to satisfy stringent requirements. They need to show excellent properties with regard to chemical and physical stability, toxicity and rheology. They are neither allowed to corrode the constituents of the printing heads, nor to clog the nozzles or to form deposits on the heating elements of thermal printing heads. On the recording sheets, however, the inks need to dry quickly and the dyes are, for example, not allowed to diffuse, they need to be light stable and they are not allowed to deteriorate or bleach when the printed images are stored in contaminated air. Furthermore, they are not allowed to reduce the gloss of the recording sheets or to disturb the process of lamination of such printed recording sheets. The choice of suitable dyes for high quality ink jet printing is strongly limited by these requirements. Only a few of all known dyes more or less fulfill all the requirements asked for in high quality ink jet printing. Probably no dye exists that fulfills all these requirements completely.
The best water-soluble magenta dyes for ink jet printing are among the group consisting of γ-acid azo dyes, H-acid azo dyes and copper complex azo dyes.
Metal free azo dyes with an incorporated y-acid moiety in their molecular structure are described for example in patent applications WO 96/24,636, EP 0,755,984 and EP 1,160,291. These dyes show a very good light fastness on polymer based recording sheets. On nanoporous recording sheets, however, their light fastness is not very good and their stability in contaminated ambient air is insufficient.
Metal free azo dyes with an incorporated H-acid moiety in their molecular structure are described for example in patent applications EP 0,918,074, EP 1,219,682 and EP 1,367,098. The most brilliant magenta dyes are found in this class. On nanoporous recording sheets, these dyes show good light fastness as well as good stability in contaminated ambient air. However, on polymer based recording sheets, their light fastness is lower in comparison with magenta dyes with an incorporated γ-acid moiety in their molecular structure or with the copper complex azo dyes mentioned below.
Metal complex azo dyes may also be used. Copper is the preferred metal. Such copper containing dyes are described for example in patent applications EP 1,086,993, EP 1,086,999 and EP 1,403,328. These dyes show good light fastness on polymer based recording sheets and on nanoporous recording sheets. On nanoporous recording sheets, they show a good stability in contaminated ambient air. However, their brilliance is lower in comparison with magenta dyes having a γ-acid moiety in their molecular structure or having a H-acid moiety in their molecular structure.
Nickel complex dyes are described in patent application EP 0,902,064. These dyes, however, are questionable from an ecologic or toxicological point of view.